Method and apparatus for isolation of flux materials in flip-chip manufacturing

ABSTRACT

A preformed planar structure is interposed between the chip(s) and the substrate in a flip-chip structure, and establishes a minimum gap between the chip(s) and the substrate. Liquid flux may be applied to the preformed planar structure in order that flux is selectively applied to the solder balls (pads) on the chip and the substrate. The preformed planar structure may be provided with through holes in registration with the solder balls on the chip(s) and the substrate. In this case, liquid flux selectively fills the through holes for delivery to the solder balls during soldering. The through holes also aid in maintaining registration of the chip(s) and the substrate. The through holes may be sized to establish a predetermined mechanical structure of solder joints formed by the solder balls when fused together. The preformed planar structure has a planar core and opposing planar faces. The core is formed of thermosetting organic resin, such as polyimide, or non-organic material such as alumina, polished sapphire, beryllium oxide, aluminum nitride or aluminum. The planar faces of the preformed planar structure are formed of thermoplastic resin or thermosetting material, such as polyacetal, expoxide resin or polystyrene. The preformed planar structure tends to draw the chip(s) together to the substrate, establishing a flip-chip structure of mechanical integrity. The preformed planar structure has a thickness of 5-50 microns, preferably on the order of 20-30 microns. Method and apparatus are disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division of copending U.S. patent application Ser. No. 576,182filed Aug. 30, 1990, now U.S. Pat. No. 5,111,279, which is acontinuation of Ser. No. 400,572 filed Aug. 28, 1989, now abandoned.

TECHNICAL FIELD OF THE INVENTION

The invention relates to semiconductor "flip-chip" manufacturingtechniques and, more particularly, to the fluxing and soldering stepsemployed in flip-chip manufacture.

As used herein, the term "substrate" refers to a semiconductor (silicon)chip to which one or more semiconductor chips are soldered inface-to-face relationship. The resulting assembly is termed a flip-chipstructure.

BACKGROUND OF THE INVENTION

"Flip-chip" manufacturing techniques involve soldering one or moresemiconductor (silicon) chips (one is discussed), in face-to-facerelationship, to another semiconductor chip termed a "substrate".Typically, solder balls (otherwise known as pads or bumps) are formed(raised above the planar surface of the chip and substrate) on facingsurfaces of both the chip and the substrate at intended points ofcontact between the two, liquid flux (rosin) is often applied to theface of the chip and/or substrate, the chip is mechanically held inregister with the substrate, and the chip and the substrate aresubjected to elevated temperature to effect soldering, or fusion of thesolder balls on the chip and the corresponding solder balls on thesubstrate.

The "solder balls" on either the chip or substrate, typically those onthe substrate, may be solderable metallized surfaces. The solderingprocess may be carried out in a reducing atmosphere. A typical flip-chipstructure is shown in FIG. 1, and is discussed in greater detailhereinafter.

Previous systems of rigid attachment of chips to chucks have been usedfor chip alignment, but they must allow some degree of compliancebecause the chips tend to change relative alignment during soldering bysurface tension between the solder balls. The addition of liquid flux tothe chip/substrate (flip-chip) assembly creates capillary attractionbetween the chip and the substrate, which serves to misalign the chipwith respect to the substrate. This is illustrated in FIG. 2, and isdiscussed in greater detail hereinafter. Further, much of the flux thatis applied to the flip-chip assembly is wasted. Still further, thedimension of the remaining gap between the chip and the substrate andthe mechanical properties of the solder joints formed by the solderballs and corresponding solder balls tends to be indeterminate.

DISCLOSURE OF THE INVENTION

It is therefore an object of the invention to provide a flip-chipmanufacturing technique which reduces capillary action, and hencemisalignment, between a chip and a substrate.

It is a further object of the invention to provide a flip-chipmanufacturing technique which requires the use of less flux, and whichcontrols the position of the flux between the chip and the substrate.

It is a further object of the invention to provide a flip-chipmanufacturing technique which provides a controlled spacing betweenchips and the substrate.

It is a further object of the invention to provide a flip-chipmanufacturing technique which provides solder joints having predictableand tailorable mechanical characteristics.

It is a further object of the invention to provide a flip-chipmanufacturing technique that simplifies the face-to-face joining of thechip and substrate.

According to the invention, a preformed planar structure is interposedbetween the chip(s) and the substrate in a flip-chip structure. Thepreformed planar structure establishes a minimum gap between the chip(s)and the substrate.

According to a feature of the invention, liquid flux is applied to thepreformed planar structure in order that flux is selectively applied tothe solder balls (pads) on the chip and the substrate.

In an embodiment of the invention, the preformed planar structure isprovided with through holes in registration with the solder balls (pads)on the chip(s) and the substrate. In this embodiment, liquid fluxselectively fills the through holes for delivery to the solder ballsduring soldering. The through holes also aid in maintaining registrationof the chip(s) and the substrate.

According to an aspect of the invention, the through holes are sized toestablish a predetermined mechanical structure of solder joints formedby the solder balls when fused together.

According to an aspect of the invention, the preformed planar structurehas a planar core and opposing planar faces. The core is formed ofthermosetting organic resin, such as polyimide, or non-organic materialsuch as alumina, polished sapphire, beryllium oxide, aluminum oraluminum nitride. The planar faces of the preformed planar structure areformed of thermoplastic resin or thermosetting material, such aspolyacetal, epoxy (epoxide resins) or polystyrene. The preformed planarstructure tends to draw the chip(s) together to the substrate,establishing a flip-chip structure of mechanical integrity.

According to an aspect of the invention, the preformed planar structurehas a thickness of 5-50 microns, preferably on the order of 20-30microns.

Other objects, features and advantages of the invention will becomeapparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a typical, prior art flip-chipstructure.

FIG. 2 is a cross-sectional view of a prior art flip-chip assemblyillustrating capillary action caused by liquid flux, resulting in themisalignment of a chip with respect to a substrate.

FIG. 3 is an exploded cross-sectional view of a flip-chip assembly,prior to soldering, according to the technique of the present invention.

FIG. 4 is a perspective view of a plastic standoff element (preformedplanar structure) employed in the technique of FIG. 3.

FIG. 5 is a perspective view of an alternate embodiment of a standoffelement suitable to be employed in the technique of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 illustrate the prior art technique of assemblingflip-chips 10. The completed flip-chip structure 10 includes one or moresilicon chips 12 (two of such chips are illustrated) mounted inface-to-face relationship to a larger silicon chip or substrate 14 inthe following manner. Solder balls (pads) 16 are formed on the face 12Aof the chip 12, and solder balls (pads) 18 are formed on the face 14A ofthe substrate 14 in corresponding positions. It should be understoodthat the solder balls on either of the chip 12 or substrate 14,typically the corresponding solder balls 18 on the substrate 14, may besolderable metallized surfaces.

Liquid flux 20 (shown in FIG. 2 only) is applied to the face 14A of thesubstrate 14, prior to bringing the chips 12 in face-to-facerelationship with the substrate 14. The chips 12 are placed inface-to-face relationship by mechanical means, such as with a chuck (notillustrated) and the temperature of the chips and substrate is elevatedsufficiently to cause the solder balls 16 on the face 12A of the chips12 to "fuse", forming solder joints, with the corresponding solder balls18 on the face 14A of the substrate.

As illustrated in FIG. 2, the liquid flux 20 fills a gap 22 between thefaces 12A of the chip 12 and the face 14A of the substrate 14, and alsofills an area on the face 14A of the substrate 14 between the chips 12.Capillary action and/or surface tension created by the presence of theliquid flux 20 causes the chips 12 to migrate to the center 24 of thesubstrate 14 during the soldering process, resulting in a lack ofregistration between the chips 12 and the substrate 14, and hencebetween the solder balls 16 and the corresponding solder balls 18,respectively. This tendency of the chips to become misaligned duringsoldering is augmented by any initial lack of planarity between the chip12 and the substrate 14, and is extremely disadvantageous in theassembly of flip-chips. For instance, it becomes extremely difficult tocharacterize the mechanical configuration of the solder joints not onlybecause of the difficulty in maintaining accurate registration, but alsobecause of the somewhat unpredictable dimension of the gap 22 betweenthe chips and the substrate. This can adversely affect flip-chipthroughput in the manufacturing process. Further, as is evident fromFIG. 2, much more flux is used than is required to effect clean solderjoints between the solder balls 16 and corresponding solder balls 18,respectively.

FIG. 3 illustrates the present, inventive technique of assemblingflip-chips 30. In a manner similar to that previously discussed, theflip-chip assembly 30 includes one or more silicon chips 32 (two of suchchips are illustrated) ultimately mounted in face-to-face relationshipto a large silicon chip or substrate 14 in the following manner. Solderballs 36 are formed on the face 32A of the chip 32, and solder balls 38are formed on the face 34A of the substrate 34 in correspondingpositions. Notably, liquid flux is not applied to the face 34A of thesubstrate 34, prior to soldering the chip 32 thereto. Nor is liquid fluxrequired to be applied to the faces 32A of the chips 32.

Prior to soldering the chips 32 to the substrate 34, a performed planarstructure 40 (otherwise termed a "stamp" or "plastic standoff element",and discussed in greater detail hereinafter), of similar planardimension as the chip 32, is interposed between the chips 32 and thesubstrate 34. The planar structure 40 is provided with through holes 42in positions corresponding to the positions of the solder balls 26 and38, respectively. Inasmuch as the solder balls 36 are typically locatedjust within the perimeter of the chips 32, the through holes 42 would belocated just within the perimeter of the planar structure 40.

Prior to soldering, the planar structure 40 is dipped (not illustrated)into a solution (bath) of liquid flux, such as rosin material, and isallowed to dry, as shown at 44. In this manner, the planar structure 40receives selectively deposited rosin preferentially within the holes 42,in registration with the corresponding solder balls 32 and 34, and therewill be be very little, if any, flux on the planar surface of thepreformed planar structure. The capaillary action of liquid solutions insmall holes draws the bulk of the liquid flux material into the throughholes 42, which are disposed in register with the solder balls, toeffect successful solder bonding.

The selective application of flux to the solder balls has numerousadvantages: there will be very little flux on the surfaces of the planarstructure; a minimum amount of rosin is used to flux the solder balls;extra (waste) flux is kept out of the flip-chip assembly; and relativemotion of the chips otherwise caused by the capillary action and/orsurface tension of such excess molten flux will be minimized.

By successive dilution of the flux bath into which the planar structureis dipped, an optimal amount of flux can be empirically determined forparticular applications that will both successfully flux the solderballs and minimize flux usage, and the aforementioned problems inherenttherein.

Thus, the chips 32 are more easily and accurately held in place bymechanical means, such as with a chuck (not illustrated) duringsoldering, resulting in increased throughput (yield) of flip-chips inthe manufacturing process. Further, the holes 42 in the planar structure40 assist in maintaining registration of the solder balls 36 andcorresponding solder balls 38, respectively, and hence alignment of thechips 32 with respect to the substrate 34.

FIG. 4 shows, in further detail, the preformed planar structure 40 usedin the inventive technique of FIG. 3. The planar structure 40 includes aplanar core 46 formed of a material such as thermosetting organic resinor non-organic material (e.g. aluminum sheet, alumina sheet, berylliumoxide sheet). Laminated to both opposing faces of the core 46 are planarlayers (faces) 48 formed of thermoplastic (resin) or thermosetting"skin" which can be expected to soften significantly at the elevatedtemperatures employed for solder reflow in the flip-chip bondingprocess. This softening and consequent shrinking of the thermoplasticresin skin (and hence shrinkage of the overall planar sturcture) willallow and encourage the chips to draw or grow closer to the substrate inresponse to surface tension caused by the molten solder balls andsurface tension of the skin itself. This "growing together" of the chipsand substrate, in other words diminution of the gap therebetween, isdesirable to ensure that each and every solder ball has an opportunityto grow together and successfully fuse. Again, this enhances flip-chipthroughput (yield) in the manufacturing process. It is also important toregulate the amount of growing together in that the requirements forsolder ball shape may require a planar structure which does not generatea structure of minimum surface area.

The thermoplastic faces 48 will resolidify after soldering (uponreduction in temperature) and create a cushion for the faces 32A ad 34Aof the chips 32 and substrate 34, respectively. Simultaneously, theshrinkage of the planar structure 40, especially the thermoplastic faces48 thereof, will have the effect of drawing the chips together as theycool off to room temperature after soldering. In this manner, the solderballs are mechanically kept in contact with the chips and substrate,respectively, as well as with each other.

The core 46 of the preformed planar structure exhibits good thermalconductivity, and is formed of a rigid thermosetting organic resin ornon-orgaic material, such as polyimide, polished alumina, polishedsapphire, beryllium oxide, aluminum or aluminum nitride. A suitablepolyimide is available from CIBA-CEIGY Corporation, Santa Clara, Calif.in their Probimide™ 300 or 400 Series, or Selectilux™ HTR 3,microelectronic materials. The faces 48 of the preformed planarstructure 40 are formed of a thermoplastic material such as polyacetal,epoxides or polystyrene. It is advantageous that the preformed planarstructure exhibit hermeticity and that it does not wick the liquid flux.The overall thickness of the preformed planar structure 40 is on theorder of 5-50 microns, preferably 30--30 microns, and the preformedplanar structure acts as a physical barrier standoff between the chipand the substrate.

A synergistic effect results from the use of the preformed planarstructure which effectively eliminates flux from the faces of the chipsand substrate by selectively causing the flux to be deposited on thesolder balls and corresponding solder balls, respectively. Inasmuch asthe faces of the chips and substrates are relatively clean, any adhesionof the planar structure (notably the "skin") thereto effects amechanical connection of the solder balls and corresponding solder ballsirrespective of soldering. (Albeit; the adhesion and shrinkage becomeeffective at the elevated temperature experienced during soldering).This satisfies the adage that, "good mechanical joints lead to goodsolder joint".

The preformed planar structure 40 serves as a plastic standoff elementto determine the size of the gap between the chips and the substrate.Evidently, the relatively solid core 46 of the planar structure 40 setsa relatively rigid lower limit on the amount that the chips can grow(draw) together to the substrate as the solder balls and correspondingsolder balls melt and fuse together.

As mentioned hereinbefore, in prior art flip-chip manufacturingtechniques the mechanical properties of the solder joints remainsomewhat indeterminate. By use of the planar structure 40 in themanufacturing process, flip-chip structures can be formed without theusual concerns about solder ball bond rigidity. Evidently, the throughholes 42 form a generally cylindrical "mold" of predetermined dimensionwherein the solder joints are formed. One may view the resulting solderjoint formed therein by the solder balls and corresponding solder ballsas a mechanical structure (not illusttrated) of predetermined dimension,and calculate the resulting mechanical properties thereof, such asrigidity (or elasticity), shear strength, tensile strength, bendingmoment, etc.

FIG. 5 is a perspective view of an alternate embodiment of a preformedplanar structure, or standoff element 50, suitable to be employed in thetechnique of FIG. 3. With respect to the materials used to form the core56 and faces 58, and the thickness thereof, the standoff element 50 issimilar to the standoff element 40 shown in FIG. 4. However, rather thanhaving through holes 42 in alignment with the solder balls 32 andcorresponding solder balls 34, the standoff element 50 may be providedwith corner cutouts 52 in alignment with the solder balls 32 andcorresponding solder balls 34, and is sufficiently sized so that thesolder joints are formed just outside its perimeter. The surfaces of thecorner cutouts 52 can be left relatively rough (as compared with thefaces 58) in order that liquid flux 54 tends to adhere thereto (asopposed to draining off the faces 58). As discussed with respect to theplanar structure 40, the liquid flux is applied to the planar structure50 by dipping the planar structure in a bath of liquid flux which isallowed to dry thereon. Under the elevated temperatures employed forsolder bonding, the flux will be delivered to the solder balls 32 and tothe corresponding solder balls 34.

The advantages of the standoff element 50 are similar to those of thestandoff element 40 with respect to forming a gap of predetermineddimension between the chip and the substrate, aiding in drawing togetherthe chip and the substrate, mechanically drawing together the solderballs and the corresponding solder balls, requiring less flux to effectsoldering and, to a lesser extent aiding in maintaining alignment of thechip and the substrate and alleviating the usual concerns about solderball bond rigidity.

The preformed planar (layered) structure cushions and draws and holds(upon resolidifying) the chip and the substrate together in theflip-chip manufacturing process. This improves the mechanical integrityof the flip-chip assembly and increased the resistance thereof to lossof electrical contact between the solder balls. This is important inthat the solder balls themselves serve as the mechanical point ofattachment between the chip and the substrate. Chip (to substrate) drawtogether is controlled, and a permanent tension is created between thechip and substrate.

The invention solves the problem of using too much flux and having theposition of the chips change during bonding (soldering). The inventionallows the use of an absolute minimum of flux so subsequent cleaning ofthe flip-chip assembly is simplified.

What is claimed is:
 1. A flip-chip structure comprising:a chip havingfirst solder balls on a face thereof; a substrate having correspondingsecond solder balls on a face thereof; and a separate and distinctpreformed planar structure disposed between the chip and the substrate,having a periphery and two opposite faces, one face in contact with thechip and the opposite face in contact with the substrate; cutouts in theperiphery of the preformed planar structure, extending from the one faceto the opposite face; and solder joints consisting essentially of thefirst solder balls and corresponding second solder balls fused to oneanother within the cutouts; wherein the cutouts are sized to establish apredetermined mechanical structure of the solder joints formed by thefirst solder balls and the second corresponding solder balls.
 2. Aflip-chip structure according to claim 1, wherein the preformed planarstructure has a planar core with two opposing faces and layers ofmaterial formed on the two opposing faces of the planar core.
 3. Aflip-chip structure according to claim 2, wherein the layers of materialfunction as means for drawing the chip together to the substrate whenthe chip and the substrate are subjected to elevated temperatures foreffecting soldering of the first solder balls to the correspondingsecond solder balls.
 4. A flip-chip structure according to claim 2,wherein the core is formed of thermosetting organic resin.
 5. Aflip-chip structure according to claim 2, wherein the core is formed ofnon-organic material.
 6. A flip-chip structure according to claim 2,wherein the core is formed of polymide.
 7. A flip-chip structureaccording to claim 2, wherein the core is formed of polished alumina. 8.A flip-chip structure according to claim 2, wherein the core is formedof polished sapphire.
 9. A flip-chip structure according to claim 2,wherein the core is formed of beryllium oxide.
 10. A flip-chip structureaccording to claim 2, wherein the core is formed of aluminum nitride.11. A flip-chip structure according to claim 2, wherein the core isformed of aluminum.
 12. A flip-chip structure according to claim 2,wherein the layers are formed of thermosetting material.
 13. A flip-chipstructure according to claim 2, wherein the layers are formed ofthermoplastic resin.
 14. A flip-chip structure according to claim 2,wherein the layers are formed of polyacetal.
 15. A flip-chip structureaccording to claim 2, wherein the layers are formed of epoxide resin.16. A flip-chip structure according to claim 2, wherein the layers areformed of polystyrene.
 17. A flip-chip structure according to claim 1,wherein the preformed planar structure has a thickness establishing apredetermined gap between the chip and the substrate.
 18. A flip-chipstructure according to claim 17, wherein the thickness of the preformedplanar structure is 5-50 microns.
 19. A flip-chip structure according toclaim 18, wherein the thickness of the preformed planar structure is20-30 microns.
 20. A flip-chip structure according to claim 1, furthercomprising:flux disposed within the cutouts.